1. Field of the Invention
The present invention relates generally to telecommunications and more particularly to a business mobile telephone communication system that can be equally successfully used indoors and outdoors in circumstances where the indoor communication takes place in a plant such a semiconductor manufacturing plant.
2. Description of the Related Art
Providing dependable telephone service in industrial environment is of paramount importance and directly affects efficiency of an industry. Currently known for the use for communication needs at semiconductor plants are DECT (Digital European Cordless Telecommunication) system and PHP (Personal Handy Phone) system, also known as PHS. DECT, based on a microcellular radio communication system, represents a wireless standard based on time division multiple access (TDMA). It is employed for limited-range (up to several hundred meters) wireless services. The system uses advanced TDMA technology and frequency range of 1880–1900 MHz. Two basic parts of the system are mobile handsets and fixed base stations. Transmitting bursts of data in the defined time slots provides a connection. Duplex operation uses a pair of evenly spaced slots, one for transmitting and the other for receiving. However, DECT phones cannot be successfully employed at a semiconductor plant. The reason for that will be explained below in more detail.
PHS, first launched in 1995, is a second-generation cordless telephone system proposed as a successor to the conventional analog cordless telephones. It is a full duplex wireless communication system of a micro-cell architecture using 1.9 GHz frequency band. PHS can be applied to a public mobile communication system, or to a Wireless Local Loop (WLL) that substitutes the existing telephone twisted pair to provide subscribers with telephone services, or to a wireless PBX or a cordless telephone system, or to transceivers that enable personal stations (PSs) to communicate with each other. PHS can implement a high quality voice equivalent to that of existing wired telephone through the employment of 32 kbps ADPCM voice coding. It has long standby and talk time because PHS cells cover a very small area, meaning the cells and personal stations consume low amounts of power. The system employs advanced encryption technology. Initially, it could not be employed when a user moved from one cell to another since it lacked a cell-handoff. Current versions of the PHS handsets overcame this drawback.
Specificity of some industrial products, processes and the conditions under which they are realized is such that it can adversely affect quality of telecommunication service. Clean rooms in semiconductor plants may present an example of this type of environment. Semiconductor devices are manufactured, as is well known, within a clean room having installed various manufacturing apparatuses used in various manufacturing processes (wet process, dry process, oxidation, lithography, ion implantation, etc.) and measuring apparatuses used for various measuring processes in testing for quality control. Clean rooms are used mainly in semiconductor device manufacturing facilities to improve yield by reducing contamination due to causes such as airborne particles. The maximum particle count permissible in such clean rooms must meet very exacting requirements, and much effort is devoted to that end, including the use of a highly filtered airflow through the room and the use of a pressure differential between the room and the ambient. Typically, filtered air enters the room through ceiling registers and exits through perforated floor panels forming a part of a raised (double) floor. The panels sit on adjustable height posts. The floor typically uses some perforated and some solid panels. Dampers or some other means can be used to regulate airflow. For example, a perforated panel can use a double layer of perforated sheet, and a movable register can be mounted between the two sheets to regulate airflow.
Quality of telephone communications from/to a clean room can be compromised by metal inclusions in floor panel and wall structures. Pipe frames in panels are welded at the corners, on top of which is also welded, e.g. tack welded, a perforated metal sheet. Inside the frame, parallel to two of its sides, rectangular or L-shape cross bars are welded at their end to the other two sides and to the perforated sheet metal above. Together with metal structured walls, a typical clean room thus represents a shield for electromagnetic waves and thus poses a serious problem for incoming and outgoing wireless communications using DECT or PHS.
Stable communication cannot be provided because of a multi-path resulting in signal fading—a signal degradation that occurs when multiple copies of the same radio signal arrive at the receiver through different reflected paths. The interference of these signals, each having traveled a different distance, leads to phase and amplitude variations.
A specific manifestation of multipath fading called Raley fading arises from the ensemble of reflected signals arriving at the receiver antenna and creating standing waves. From the transmitting antenna, even a tightly focused radio signal scatters, or spreads out. The ground and bodies of water reflect the signal back upward, and the atmosphere reflects the signal downward. At the receiver, portions of the signal arrive at different times, as the signal has taken multiple paths of differing path lengths from transmitter to receiver. Occasionally, the aggregate signal from the indirect paths can be of similar strength to the signal from the direct path. If the two signals are of opposite phase, a standing wave is created, and the signal fades in overall strength. The ultimate impact is that of increased transmission errors. The radio signal processing in both the base station and mobile units have to be designed to tolerate a certain level of multipath fading. One of the examples where poor quality of telecommunications especially matters and is specifically sensitive is when processing tools and equipment are not placed in the clean room area yet, and low telecommunication efficiency slows down the tool move. By and large, it reduces flexibility customarily provided by mobile telecommunication.